CAIRO—At least 74 people were killed and 748 injured in early morning fighting between police and supporters of ousted President Mohammed Morsi, the Ministry of Health said, as Egypt's political divisions edged toward prolonged conflict.

Saturday morning's violence was the deadliest single crackdown in the more than two years since Egypt's first revolution. The killings mark a dangerous escalation in a conflict that has already badly damaged Egypt's emerging democracy and ruptured a society that had once prided itself on...